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  #1  
Old 03-30-2015, 09:40 AM
Mark Dickens's Avatar
Mark Dickens Mark Dickens is offline
 
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Default RV-8 Cowling Fitment

I'm working on fitting the cowling and started with the upper cowling, which I've positioned to my satisfaction and trimmed to the firewall. I've installed the skybolts and am happy (perhaps because ignorance is bliss) with how it hangs and how it's positioned relative to the spinner backplate (FP prop). I have not yet done any trimming on the horizontal.

The issue is with the bottom cowling, which fits to the upper cowling like high heels fit on a horse. To say that Van's fiberglass work is not up to their aluminum craftsmanship is a vast understatement and an old story. I could not find a way to fit the bottom cowling to the top cowling while mounted on the plane, so I removed it and began working with both cowlings on the shop floor per Dan Horton's recommendation (http://www.vansairforce.com/communit...t.php?p=461049). I have fitted the halves together so that the opening is 13" in diameter and have the back of the cowling matched to the height of the firewall. The inlets are equal in height as well.

Pics of current status:







My questions for the cowling gurus are:

1. Should I go ahead and trim the upper and lower cowling to each other while off the plane? If I don't do that, I don't see how I can match them up on the plane.

2. I am tempted to cut off the overlapping areas of the inner inlet sections because they fit so poorly and were so sloppily made. I'm thinking I can recreate that section later.

Thanks for any thoughts!
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  #2  
Old 03-30-2015, 12:11 PM
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DanH DanH is online now
 
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Mark, you're a bit off track, but hey, it's fiberglass. It's hard to make a mistake that can't be fixed.

From a previous post:

...trim either or both to an imaginary line which exactly bisects a 13" circle.

That means trim before clecoing them together.

Start by marking and trimming the upper cowl. Align a straightedge as shown; 6.5" at the center (half the spinner diameter), while maintaining exactly equal height on the two inlets. Mark at the four red lines. Trim just outside the line (maybe 1/16"), which leaves some room to clean up the edge with a long sanding board. At the outboard cuts, don't try to trim back along the sides of the cowl just yet. Just trim the leading edge of the inlet opening:



Now trial fit to the lower cowl, on the floor, with a spacer stick or two at the rear, and the long sides overlapped. If you're lucky, the lower cowl joggles will match the freshly cut upper cowl. By lucky, I mean won-the-lottery-lucky; you'll probably need to trim or fill to get a match.



BTW, if the joggles need a lot of work, it's easier to just cut them off, trim the lower to match the upper, then add new joggles. To do that, set up and trim the lower cowl nose pretty much as you did the upper. Cleco a temporary scrap joggle any where needed. Plenty of opportunity to fill the cleco holes later in the finishing process.

With the nose trimmed to a pretty good match, align the cowl on the airplane to trim the rear edge and the sides. Attach a steel or aluminum tab to the forward baffle attach points (1/4" screw) above each front cylinder. These support the nose of the upper cowl. The rear is supported by overlapping the boot cowl. Draw a centerline on the top rear center of the cowl. Draw a matching centerline on the top of the boot cowl. Adjust the length of the front supports so the cowl nose is dead behind the spinner and perfectly level. Install a block to set the spinner gap. Align the centerlines at the rear of the cowl. Now trim the rear edge of the cowl so it sits on your hinge line or camloc tabs. The upper cowl is now where it belongs.



Raise the lower cowl into position, install the two front clecos, and run a strap around the halves. Trim the rear edge. Now trim the sides. Start by drawing a level line back from the front of the cowl where you had already established a bisect line in the very beginnning. Trim both halves to this line.

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Last edited by DanH : 08-21-2018 at 05:54 AM.
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  #3  
Old 03-30-2015, 12:33 PM
Mark Dickens's Avatar
Mark Dickens Mark Dickens is offline
 
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Quote:
Originally Posted by DanH View Post
Mark, you're a bit off track, but hey, it's fiberglass. It's hard to make a mistake that can't be fixed.

From a previous post:

...trim either or both to an imaginary line which exactly bisects a 13" circle.

That means trim before clecoing them together.

Start by marking and trimming the upper cowl. Align a straightedge as shown; 6.5" at the center (half the spinner diameter), while maintaining exactly equal height on the two inlets. Mark at the four red lines. Trim just outside the line (maybe 1/16"), which leaves some room to clean up the edge with a long sanding board. At the outboard cuts, don't try to trim back along the sides of the cowl just yet. Just trim the leading edge of the inlet opening:



Now trial fit to the lower cowl, on the floor, with a spacer stick or two at the rear, and the long sides overlapped. If you're lucky, the lower cowl joggles will match the freshly cut upper cowl. By lucky, I mean won-the-lottery-lucky; you'll probably need to trim or fill to get a match.

If the joggles need a lot of work, it's easier to just cut them off, trim the lower to match the upper, then add new joggles. To do that, set up and trim the lower cowl nose pretty much as you did the upper. Cleco a temporary scrap joggle any where needed. Plenty of opportunity to fill the cleco holes later in the finishing process.

With the nose trimmed to a pretty good match, align the cowl on the airplane to trim the rear edge and the sides.
Aha...the part I didn't get earlier is the part about not cutting along the sides yet. Makes sense. I think the light just came on that the first step is getting the inlet portions to decently match and once that's done, you attack the sides and firewall sections.

Dan, THANKS for taking the time to explain this! Very appreciated!
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  #4  
Old 03-30-2015, 12:44 PM
Bevan Bevan is offline
 
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Quote:
Originally Posted by Mark Dickens View Post
2. I am tempted to cut off the overlapping areas of the inner inlet sections because they fit so poorly and were so sloppily made. I'm thinking I can recreate that section later.
Before you cut the entire inner overlapping area, try just trimming (thinning) the back side of the outer flange in case there's any excess material preventing them from fitting, AND

cut the corners out only of the inner flange. I found that the radius of the corners is where the mismatch is occurring. Those corners are easily replaced later. Even easier than redoing the whole flange.

Bevan
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  #5  
Old 03-30-2015, 01:10 PM
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Mark Dickens Mark Dickens is offline
 
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Quote:
Originally Posted by Bevan View Post
Before you cut the entire inner overlapping area, try just trimming (thinning) the back side of the outer flange in case there's any excess material preventing them from fitting, AND

cut the corners out only of the inner flange. I found that the radius of the corners is where the mismatch is occurring. Those corners are easily replaced later. Even easier than redoing the whole flange.

Bevan
Good point. Thanks!
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  #6  
Old 03-30-2015, 08:26 PM
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Ron RV8 Ron RV8 is offline
 
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Default For what it's worth...

Mark up the front of the bottom cowling on the floor... a-la Dan... Leave the sides of the bottom cowling full length.

Suggest getting an el-cheapo construction laser. This one was $15...



Block up your top cowling where you want it. Aircraft spruce catalogs, telephone books etc work well.. Tape it in place...



With the fuselage level, and using the laser and marks made using Dan's suggestions, finalize a level line around the curves on front of the top cowl...



Extend the line down the sides. Make sure there is enough to overlap the bottom cowl sides, I almost cut it too short...




You have left the back of the cowling un-trimmed right.

With the cowling off, stick a bunch of pieces of masking tape onto the fuselage lengthwise, overlapping the cowling rear cut line. Mark the rear cut line onto these pieces of tape, and fold them back onto themselves so that you can unroll these pieces forward over the cowling halves once installed to mark where the rear cut line should be.

Fit both halves back on the airplane, with the lower cowl side overlap under the upper cowl. Mark the lateral cut line.

Roll the pieces of tape you rolled back, forward and mark the rear cut line.

Triple check to make sure everything is right before you cut...!!!

After shot with rear SkyBolts installed...



YMMV, hope this helps...
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Last edited by Ron RV8 : 03-30-2015 at 08:38 PM.
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  #7  
Old 03-31-2015, 05:30 AM
Mark Dickens's Avatar
Mark Dickens Mark Dickens is offline
 
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Ron, thanks for the detailed response!

I have already trimmed the back of the upper cowl and mounted to the skybolts. It appears to be in the right place to me, although I have some fiberglass touch ups to do on the rear trim area.

I was scratching my head last night trying to figure out how to determine the trim line down the side of the upper cowl and then it hit me, use the center line of the fuselage as shown on drawing 19 (fuselage cutaway). So, I pulled a string from the center of the spinner backplate around the cowl and back to the rear of the fuselage to match the positioning of the center line. Voila, there's my trim line. I haven't cut anything yet as I have to double check my positioning and all levels, but I think this will work. Finally a use for drawing 19 other than to decorate my wall!
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  #8  
Old 03-31-2015, 09:20 AM
BillL BillL is offline
 
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Quote:
Originally Posted by Mark Dickens View Post
Ron, thanks for the detailed response!

I have already trimmed the back of the upper cowl and mounted to the skybolts. It appears to be in the right place to me, although I have some fiberglass touch ups to do on the rear trim area.

I was scratching my head last night trying to figure out how to determine the trim line down the side of the upper cowl and then it hit me, use the center line of the fuselage as shown on drawing 19 (fuselage cutaway). So, I pulled a string from the center of the spinner backplate around the cowl and back to the rear of the fuselage to match the positioning of the center line. Voila, there's my trim line. I haven't cut anything yet as I have to double check my positioning and all levels, but I think this will work. Finally a use for drawing 19 other than to decorate my wall!
The trim line between the upper/lower cowl is NOT PARALLEL to the longeron on the 7.

Just a warning for the 7 builders reading this thread. I made this mistake and had to do some re location of my SkyBolts.
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  #9  
Old 05-21-2015, 04:38 PM
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AAflyer AAflyer is offline
 
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Unhappy Inlets identical/parallel ?

First day of cowling work on my -8.
OK, so I followed DanH guidance to trim the leading edge of the top cowl on a line that bisects the center of the circle. Trimmed the outboard sections of the leading edge to alleviate interferrance.
When I fit the top and bottom together, I had to remove a lot of extra material on the inside of the top cowl under the spinner area in order to get that area to "nest" well. I ended up removing the interfering corresponding corners of the bottom cowl. This resulted in a good "nesting".
HOWEVER…my problem is that I don't have a true circle in the spinner area. When measured across the inside or outside diameter, it appears that I have an oval by about 3/8". This means that I will have to re-fab the joggle on the lower cowl, along with a lot of other trimming, of course. This, in turn will reduce the cross-sectional area of the inlets.

MY QUESTIONS…Is it logical to use the horizontal diameter as the correct measurement? Or is the fiberglass so sloppy that this physical distance is unreliable?
Are the left and right inlets the exact same size? Because I have asymmetrical openings now. When I measure the vertical distance of the left and right inlet openings, they are not the same. Is there a specified vertical dimension for the inlet openings?
BTW, I've walked away for the day, so this problem is NOT growing bigger as we speak.
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  #10  
Old 05-21-2015, 05:33 PM
DaAV8R DaAV8R is offline
 
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Quote:
Originally Posted by AAflyer View Post
First day of cowling work on my -8.
OK, so I followed DanH guidance to trim the leading edge of the top cowl on a line that bisects the center of the circle. Trimmed the outboard sections of the leading edge to alleviate interferrance.
When I fit the top and bottom together, I had to remove a lot of extra material on the inside of the top cowl under the spinner area in order to get that area to "nest" well. I ended up removing the interfering corresponding corners of the bottom cowl. This resulted in a good "nesting".
HOWEVER…my problem is that I don't have a true circle in the spinner area. When measured across the inside or outside diameter, it appears that I have an oval by about 3/8". This means that I will have to re-fab the joggle on the lower cowl, along with a lot of other trimming, of course. This, in turn will reduce the cross-sectional area of the inlets.

MY QUESTIONS…Is it logical to use the horizontal diameter as the correct measurement? Or is the fiberglass so sloppy that this physical distance is unreliable?
Are the left and right inlets the exact same size? Because I have asymmetrical openings now. When I measure the vertical distance of the left and right inlet openings, they are not the same. Is there a specified vertical dimension for the inlet openings?
BTW, I've walked away for the day, so this problem is NOT growing bigger as we speak.
If my cowl was trimmed such that the center hole was perfectly round there was about 1/4" to 3/8" fiberglass "land" exposed all round the spinner. I didn't like that look so I cheated the vertical dimension and trimmed the cowl so that the line of the cowl flowed into the spinner at the top and bottom when viewed from the side. The exposed land on the sides of the spinner seems less noticeable than an exposed land above and below the spinner.

I like the look of my cowl but what I don't know is whether I have reduced the inlets just a bit and whether this will cause any cooling challenges. Time will tell.

The inlet openings should be the same on each side.
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